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1 Second Quarter 2001 Monitoring and Sampling Report 2 7-Eleven Inc Store No-1 7334 4501 North Pershing Avenue Stockton CA July 9 2001 ' Monitoring Well Sampling and Analytical Results Quarterly groundwater samples were collected after purging at least 4 well-casing volumes of groundwater from each monitoring well, unless the well was purged dry The groundwater samples were collected using disposable bailers Groundwater samples were placed in analysis-specific containers with analysis-specific ' preservative The samples were then sealed using Teflon septum caps, labeled with the job identification number, the sample number,date,time, and type of laboratory analysis required The samples were stored on Ice and transported with a chain-of-custody record to a state-certified laboratory for analysis ' Groundwater samples were analyzed for BTEX and TPH-G using Environmental Protection Agency(EPA) methods 8021 B/modified 8015, and for oxygenates MTBE, TBA, DIPE, ETRE, and TAME using EPA method 8260E Analytical results for the quarterly sampling event are presented in Table 1, and historical laboratory analytical results are summarized in Table 2 Highest dissolved benzene concentrations were reported in well MW-5A at 3,100 micrograms per liter (pg/L) Dissolved MTBE concentrations were highest in wells MW-5A(2,100 pg/L) and MW-4 (3,900 pg/L) Other monitoring wells contained less than 5 pg/L MTBE, except well MW-5B which contained 18 pg/L Selected dissolved constituents are plotted on Figure 2 The ' laboratory analytical reports are included in Attachment 4 Field records indicate the presence of a possible offsite contaminant source, a remediation system was observed on the property located directly across North Pershing Avenue from the 7-Eleven site This finding may indicate plume mixing in the area of well MW-4 with another hydrocarbon source (not 7-Eleven) Other ' quarterly data obtained during this sampling event are consistent with previous findings, indicating the hydrocarbon source is in the vicinity of the underground storage tanks Dissolved plume constituent concentrations in the deeper nested well MW-513 indicate vertical plume delineation near the source area Prior to purging, dissolved oxygen (DO) levels were recorded in each well Dissolved oxygen levels indicate generally anearobic conditions(less than 2 milligrams per liter(mg/L)) (Table 1) As mentioned In previous quarterly reports, a well search database for public water supply wells located within one mile of the site Indicates that there are no public water supply wells located within 4,000 feet downgradient from the site The nearest crossgradient/upgradient water supply well of unreported status Is located more than 1,500 feet southeast of the site 17334012 0MR